Of crucial importance for the ability of a starter battery to start a motor vehicle having an internal combustion engine are the charge state and the advancement of aging or the decline in capacitance, because these limit the current strength which can be drawn from the starter battery, or the power output thereof. Starting the engine is amongst the heaviest demands on the battery in the motor vehicle. The starting procedure is when it is most noticeable if the performance of a battery is reduced by aging.
The reduction in the loading capacity of a battery over relatively long periods of time is caused, on the one hand, by the rise in the nonreactive internal resistance of the battery, and, on the other hand, by passivation phenomena on the active materials.
In a modern motor vehicle, the properties of the generator, the battery and the electrical loads are matched to one another such that only an instance of several mishandling can result in battery charge states being so low that the starting ability is no longer assured. The request for a reliable indicator for battery replacement is, therefore, of great importance, particularly when safety-related electrical loads are installed, such as electric brakes or electrically assisted steering aids.
DE-C 3901680 discloses a method for monitoring the cold starting ability of the starter battery for an internal combustion engine, in which the time profile of the voltage drop which occurs on starting is observed and evaluated. The evaluation is carried out on the basis of maximum ratings for a characteristic curve obtained from empirical values, and on the basis of the battery temperature.
DE-C 3712629 describes a measuring device for the remaining useful life of a motor vehicle battery which records the battery voltage and the associated load current value before and after starting for the first time with the battery in the fully charged state, ascertains the temperature-compensated internal resistance, stores it in a memory and compares it with the internal resistance values ascertained for subsequent starting procedures in the internal combustion engine. An indication is then given on the basis of prescribed stored threshold values.
DE-A 1 19750309 discloses a method for determining the starting ability of the starter battery in a motor vehicle, in which the voltage dip on starting is recorded and stored taking into account the temperature of the battery and the engine. The increase in the size of the voltage dip, associated with aging of the battery, as compared with the fresh values is used as a measure of battery aging.
WO 99/17128 discloses the practice of comparing, during the starting procedure, the voltage values of adjacent voltage minima arising on account of the compression and decompression of the engine pistons. The difference between these values is used as a measure for indicating the battery state.
Thus, it would be highly advantageous to provide a method for determining tie starting ability or the availability of a storage battery used for starting an internal combustion engine, wherein the method reliably ascertains the state of the battery.